Films, fibers and other shaped articles prepared from PEN display improved strength and thermal properties relative to other polyester materials. High strength fibers made from PEN can be used to make tire cords and films made from PEN are advantageously used to manufacture magnetic recording tape and components for electronic applications. In comparison with polyethylene terephthalate, PEN is excellent, for example, in mechanical strength and heat stability. PEN is used for films for magnetic tapes, for films for packaging, and for condensers. In recent years it has been used for photograph supports because of its dimensional stability in the form of a thin film.
2,6-NDA and ethylene glycol are the raw materials for producing PEN. The methods for producing PEN are the esterification process and the direct polymerization process, each of which can be carried out batchwise or continuously. Currently in the art the esterification process is more commonly used, but usually employs as the raw material a salt of 2,6-NDA, 2,6-naphthalene dicarboxylate (hereinafter abbreviated 2,6-NDC). 2,6-NDC is often in crystal form with impurities trapped in the structure. 2,6-NDC is used, because there have been no processes available in the art to produce polymerization grade 2,6-NDA, the preferred monomer for making PEN. The availability of a process for making polymer grade 2,6-NDA would make it possible to pursue the preferred route to PEN. This would represent a revolutionary advance in the art. PEN produced by such a process would be much more economical.
It is also known in the art that all previous processes for producing NDA and NDC deliver a solid product that is usually shipped to the polymer manufacturing plant. Though 2,6-NDA is the preferred monomer, handling of NDA particles is still difficult and expensive. In addition, particle size can be critical where dry handling of solids is practiced.
U.S. Ser. No. 4,755,587, for example, discusses the problem of handling solids and claims advantages using very small porous pellets.
Copending U.S. Ser. No. 60/151,577, filed of even date, and incorporated by reference herein in its entirety, discloses a process for producing 2,6-NDA of polymer grade. The new process is unique in many respects. Of particular importance, the new process can operate using relatively impure methylnaphthalene feedstock with respect to organic hydrocarbon impurities, allows for debromination of the oxidation product in the liquid phase, and avoids the isolation of purified naphthoic acid.
The advent of a process for making polymer grade 2,6-NDA, suitable for direct use in a PEN process without esterification, or solids drying and handling and the associated problems of particle size control is of tremendous value in the art. Such a process constitutes a very significant advance in the art, is much more economical, and is capable of producing polymer grade 2,6-NDA that could be slurried directly into a PEN process. This eliminates problems with solids handling, including the major expense of transporting the 2,6-naphthalene dicarboxylate solids. This constitutes a tremendous advance in the art.